Which Fuel Grade Is Right for Your Car?

The gasoline deals go by many names: Wacky Wednesday, Thrifty Thursday, Super Sunday. On specific days, some stations knock a nickel or so off the price of a gallon of premium gas. It's almost as cheap as the station's mid-grade line of gasoline.

Is premium gas a good deal? In general, no, according to the experts. Sometimes, yes, but even then it can be open to question.

This fall, the American Automobile Association (AAA) issued an advisory that found that premium gas is an expensive waste for most drivers. ''Vehicles that run well - do not knock or ping - on regular-grade gasoline will not benefit from using a higher grade of fuel,'' it concluded.

Unfortunately, many Americans buy premium unnecessarily. The federal Energy Information Agency estimates that premium will represent nearly 19 percent of gasoline sales this year. In reality, fewer than 5 percent of cars in the United States really need premium fuel, according to one estimate quoted by AAA.

Not only are consumers buying too much premium gas, they appear to be paying a high price for it. Unofficially, AAA officials say it costs 4 to 5 cents more to make a gallon of premium than a gallon of regular gasoline. But at the pump, premium costs an average 17 cents more and sometimes as much as 40 cents more, the AAA estimates.

''The oil companies have to be laughing all the way to the bank,'' says David Van Sickle, director of automotive information at AAA's national office in Washington, D.C. Of course, some of the extra cost is justified, because it's more expensive to stock and transport three grades of fuel than one. Still, Mr. Van Sickle says, ''this is a marketing triumph.''

While most cars don't need premium gas, some - especially high-performance models - can benefit from using it. Many high-performance cars require premium to operate at peak performance. These include models from Acura, BMW, Cadillac, Lincoln, Mercedes, and other luxury manufacturers.

To understand why, it helps to know something about car engines. To get more power, manufacturers sometimes build engines with a higher-than-normal compression ratio. High-performance cars often use this design because it saves space and weight.

A high-compression ratio means that each piston inside the engine squeezes the mixture of fuel and air into a smaller-than-normal space. This creates extra power when the air-fuel mixture expands. Unfortunately, it also generates extra heat.

If the air-fuel mixture gets too hot, it explodes before it's supposed to. This causes a metallic rattling in the engine that mechanics call ''knocking.'' If you hear it in your car, switch to a higher grade of gasoline. Premium gas eliminates knocking because it ignites at a higher temperature than regular gas does. So it's much less likely to explode prematurely. In general, use the type of gas your car manual recommends.

There are a couple of exceptions, however. As cars get older they become more susceptible to knocking and may require a higher grade of fuel. On the other hand, even if your car manual recommends premium, you can often get by with a lower grade. Most of today's engines have electronic knock sensors that will adjust the timing with lower-grade fuels. And even regular gasoline is now required to contain the detergent additives that control unwanted deposits inside the engine.

Even though my Volvo recommends premium gas, I've found I can skimp on that. Two months ago I switched over to mid-grade fuel when I discovered that the high-grade stuff was boosting the car's fuel efficiency by only 1 percent, while it cost 2 percent to 3 percent more to buy, even on those Wacky Wednesday discount days.

So I use mid-grade fuel exclusively now. My Wednesdays are a lot less wacky.